1. Field of the Invention
The invention relates to a method and apparatus for providing an input preprocessor function to realize the input current to voltage nonlinearity for translinear circuits (such as Gilbert amplifiers and multipliers) and other non-translinear circuits with a fixed bandwidth independent of signal level or temperature. Additionally, there is disclosed methods and apparatus for electrically adjusting the transconductance of an amplifier to allow the bandwidth of the invention to be electrically adjusted.
2. Background Art
A Gilbert amplifier is a special circuit technique that provides desirable attributes such as operation in a current gain mode with a wide bandwidth response for handling of current input signals such as photodetector signals.
FIG. 1 shows a prior art Gilbert amplifier model as disclosed in "Current-mode Circuits From A Translinear Viewpoint: A Tutorial" by B. Gilbert, pp. 11-19 and 55-64 of Ch. 2 of Analogue IC Design: the current-mode approach, C. Toumazou, F. J. Lidgey & D. G. Haigh, eds., published by Peter Peregrinus Ltd., London (1990). The photodetector pair is modeled as current sources 10 and 12 with a parallel capacitances 14 and 14'. The output current difference is proportional to the difference of the input photocurrents, with a gain factor proportional to the control current.
Ignoring second-order effects, the overall gain of this can be calculated as: ##EQU1## where: I.sub.out(differential) is the difference in output currents;
I.sub.ina and I.sub.inb are input currents; and PA1 I.sub.ctrl is the tail current.
The features of the prior art Gilbert amplifier are wideband normalization, fast overload recovery, good accuracy for DC and wideband signals, simple implementation of automatic gain control, and stability for any input condition.
The problem with the prior art Gilbert amplifier is that the bandwidth is a function of a) DC input current level; b) temperature; and c) input capacitance. ##EQU2##
Thus, bandwidth and group delay (time delay through the circuit) is dependent on the source characteristics and temperature.
Optical disk drives have a number of input signals in the form of differential input currents, for example, photodiodes. These include focus error signals, magneto optic (MO) read channel signals and actuator position signals. A normalized amplifier is typically used for noise and servo compensation reasons. Prior normalized amplifier implementations, i.e., Gilbert amplifiers, do not provide adequate bandwidth at the lower input signal levels associated with MO (erasable) drives.
U.S. Pat. No. 4,396,891 to Johansson et al. describes a gain control circuit that is related to a Gilbert multiplier, but does not address high impedance input signals or bandwidth.
U. S. Pat. No. 4,492,931 to Deweck discloses an infrared receiver front end which includes a transimpedance amplifier as opposed to a translinear amplifier (Gilbert). Deweck discloses a preamplifier for interference rejection and does not deal with bandwidth.
U.S. Pat. No. 4,502,017 to Van de Plassche et al. discloses an operational amplifier, however, it is not usable with translinear circuits.
U.S. Pat. No. 4,608,542 to Siegel, like Deweck, describes a bandwidth control of transimpedance amplifiers, however, it is not usable with translinear amplifiers.
U.S. Pat. No. 4,902,982 to Moore et al. describes a nonlinear noninverting transimpedance amplifier that uses a nonlinear feedback structure to prevent overload of high dynamic range signals. This circuit is not translinear and the device does not deal with bandwidth controls.
U.S. Pat. No. 5,254,957 to Lauffenburger is the most analogous reference. The present invention overcomes shortcomings of Lauffenburger which are bandwidth variations with temperature and signal level.
In accordance with the present invention there is provided an apparatus for amplifying and processing high-impedance current mode input signals that provides input normalization, DC accuracy and wide band signal handling.
The preferred input preprocessor provides nonlinear transformation of a current mode input signal for a single input Gilbert amplifier. The processor reduces bandwidth dependency on the input current. The preprocessor is comprised of a common base transistor which acts as a feedback element for nonlinear conversion of the input current to a voltage. A transconductance element is applied to the transistor for setting a bandwidth for the input preprocessor.
The apparatus can also comprise multiple preprocessors with multiple current inputs. In this embodiment, the preferred translinear circuit is a differential Gilbert amplifier. The apparatus may further comprise at least one replicating transistor for replicating a current through the input preprocessor. The replicating current mirror comprises transistors with common emitters and common bases with the common base transistor.
The method is also disclosed for reducing bandwidth dependency on an input current for nonlinear current to voltage transformations. The steps comprise first providing a common base transistor as a feedback element for nonlinearly converting an input current to a voltage. A transconductance element is then applied to the feed back element for forward gain.
The method may also include the step of providing multiple preprocessors for multiple current inputs. In these steps, a multiple input differential Gilbert Amplifier is provided.
A primary object of the present invention is to provide all the advantages of a Gilbert amplifier for a low-current source.
A primary advantage of the present invention is that it provides a simple circuit implementation that achieves the desired goals of: a) operating with a low input bias current; b) wideband normalization of the input signal; c) fast overload recovery; d) good accuracy for DC and wideband signals; e) simple implementation of automatic gain control; and f) stability for a wide range of input conditions.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.